Router web UI slow and unresponsive on modern macOS (and iOS), but super snappy on OSX 10.4

I have a router running the FreshTomato firmware. If I connect to the web UI in any of the following ways, it is very laggy (sometimes 10+ seconds for a simple page navigation): - 5k iMac connected via a gigabit ethernet switch - the same 5k iMac connected directly over ethernet - M1 Air wirelessly - iPad Pro If I dust off an old 12" G4 Powerbook and repeat the first three connections, it is in another league. Super-snappy, every time. In all cases, connections *through* the router to other web interfaces seem fine. It's just modern Safari and FreshTomato that don't agree. Chrome seems a *little* better, but still not a match for vintage OSX 10.4. I don't have any content blockers or extensions installed. What is likely to be the cause? # Update The interface is really quite simple, which made me think it unlikely that this really was a browser problem. Connecting via Proxyman reported a high rate of errors (NIOCore.ChannelError). I was reluctantly coming to the conclusion that this was going to descend into Wireshark territory (maybe it still will!), but I thought I'd try something else first... If I route traffic via an intermediate machine (ssh -L 8080:tomato:80 pi and point Safari at 127.0.0.1:8080), suddenly I get the same snappy experience my Powerbook G4 enjoys. My low-level networking isn't strong enough to spot the true cause / solution though. Achieving a similar result via an nginx instance has the same positive result: location / { proxy_pass http://tomato/ ; } I don't consider needing a 3rd device in order to manage a router as a final solution.